[AVR:] is a valid topic tag by the way

http://www.piclist.com/#topics

> The most likly cause of your problem is short term power loss
> from intermitant connections to the track rails. (a few mS)

If the cars are anything like the ones I used to have (Scalextric)
those motors are excellent little noise generators. Snow on TVs
for miles around. It's possible that EMF is resetting the AVR as
well as the power supply dipping

You might want to have a look and see if there are any caps on
the motors. A 10nF ceramic from each terminal to the case (is
it possible to ground the motor case ?) and 10nF across the
terminals. And two zeners in series but opposite polarity --><---
across the terminals too. If you can use these small components
inside the car to cut the noise down you might not need to use
big ones like caps outside

Ditto for what Denny said about isolating the AVR's power cap
with a diode

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